The long-term objective of the project is to determine how visual information is used to control human locomotion in a complex, changing Environment: What information is used to perceive properties such as one's direction of heading, collisions with stationary and moving objects, and how is this information used for posture and gait. The proposed research will investigate interactions between multiple control laws for balance, steering, and obstacle avoidance. In previous work it was found that the visual system accurately recovers heading form optic flow under a variety of conditions. It was also found that posture is adaptively regulated by optic flow during walking, and is closely integrated with gait. The aim of the proposal is to dynamically model interactions for control strategies for balance, steering, and obstacle avoidance. Virtual reality techniques will be used to dissociate hypothesized strategies in treadmill walking and in joystick tasks. Five projects are proposed: 1. Interaction of the focus of expansion and visual direction strategies for steering. 2. Interaction of the focus of expansion and equalization strategies for steering. 3. Integration of goal seeking and obstacle avoidance. 4. Perception of heading and active steering. 5. Integration of posture, gait, and steering. The results will contribute to basic knowledge about visual control of locomotion and provide a foundation for clinical research on visual- motor deficits, gait disorders, and mobility problems in disease and aging.